Changes in photoperiod and nutrition and their effect on testicular growth of rams.

Groups of 6 Ile-de-France rams were housed in light-proof rooms and subjected in a factorial design to two light regimens, 180 degrees out of phase, and two levels of protein in their diets. The daily duration of daylight was varied sinusoidally to produce 6-month years with 'winters' of 8 h light and 'summers' of 16 h light. The diets were formulated to supply 50% above or 25% below maintenance requirements in protein. Testicular diameter and volume increased with decreasing light and decreased with increasing light but the diet had no effect. The frequency of LH pulses was measured monthly and was high (3/12 h per ram) when the daylight was being reduced and low (1/12 h per ram) when it was increased. At the extremes of the duration of dark or light the frequency of pulses was around 1.6/12 h per ram, regardless of the duration of light. The two diets had no effect on testicular dimensions but rams fed the 'high' protein diet had a total of 175 LH pulses, which was significantly higher (P less than 0.05) than the 131 pulses recorded from rams on the 'low' protein diet. It is concluded that, in these '6-month years', decreasing light stimulates LH pulsatility and testicular growth and increasing light is inhibitory. Pulsatility of LH appears to be influenced by the protein level in the diet.